(platenioGH-35) Implement the getCssClass shortcode

This commit implements a helper shortcode, `getCssClass` to be used in
our patch on the `_default/baseof.html` template to retrieve the class
or classes which should be injected into the `body` of a page.

It defaults to returning an empty string.

In this implementation, If the page is a member of a top-level docs
section (`docs` by default, but can be configured to `games` or anything
else via the `Params.BookSection` parameter in the site config), such as
`content/docs/my_game/rules.md` or `content/games/my_game/rules.md`, then:

1. If the `_index.md` for that section does **not** define `cssClass` in
its frontmatter, then the default CSS class injected for that section is
the URL segment for that section; in this example, `my_game`.

2. If the `_index.md` for that section **does** define `cssClass` in its
frontmatter, then the CSS class injected for that section is the value
of that parameter. This can be used to inject multiple classes, each of
them separated by a single space.

In this current implementation, other top-level pages and sub-sections of
a given section may not override or append their own class injections.

layouts/partials/platen/getCssClass.html

@@ -0,0 +1,32 @@
+{{/*
+    This injects a class based on either cssClass in the _index.md or the filepath.
+    (The cssClass override is especially useful when the filepath name isn't a valid CSS class name)
+*/}}
+{{- $injectedClass :=  "" -}}
+{{- $currentLink := .Permalink -}}
+{{- $currentParams := .Params -}}
+{{- $bookSection := default "docs" .Site.Params.BookSection -}}
+{{- if eq $bookSection "*" -}}
+    {{- $bookSection = "/" -}}
+{{- end -}}
+{{- with .Site.GetPage $bookSection -}}
+    {{- range (where .Pages "Params.bookhidden" "ne" true) -}}
+        {{- $page := . -}}
+        {{- $filePath := replace .File.Path "\\" "/" -}}
+        {{- $parts := split $filePath "/" -}}
+        {{- $section := index $parts 1 -}}
+        {{- $section = replace $section ".md" "" -}}
+        {{- if in $currentLink $section -}}
+            {{- with $currentParams.cssClass -}}
+                {{- $injectedClass = $currentParams.cssClass -}}
+            {{- else -}}
+                {{- with $page.Params.cssClass -}}
+                    {{- $injectedClass = $page.Params.cssClass -}}
+                {{- else -}}
+                    {{- $injectedClass = $section -}}
+                {{- end -}}
+            {{- end -}}
+        {{- end -}}
+    {{- end -}}
+{{- end -}}
+{{- return $injectedClass -}}